Control circuits are known for controlling and synchronizing an application process (e.g. cutting, sealing, perforating, printing) with respect to a moving web to which the application process is directed. One such application process involves the art of applying a hot melt material to a moving web, such as raw paper or label stock, using a hot melt material applicator such as a rotary screen coater. The Nordson rotary screen coater, manufactured by the assignee of the present invention, perform such a hot melt material application process.
To effectively perform such a process, a control circuit is required to correctly position the rotary applicator with respect to the moving web. The control circuit controls the speed of a slave process (such as the speed of a rotary applicator) based on a sensed speed of a master process (such as the speed of a web) and a selected relationship of desired slave speed with respect to master speed. Typically, motors are used to drive both the master process and slave process, and tachometers or encoders are mounted on each motor to provide an output signal representing the respective speeds of the master and slave processes.
A controller compares a reference signal (master tachometer or encoder output) with a fixed setpoint representing a desired slave/master speed ratio to determine an appropriate slave speed based on this comparison. The controller output is a control signal to the slave process motor which corresponds to the determined slave speed. This control signal output is altered in real time based on a feedback signal (slave tachometer or encoder output) which indicates the speed of the slave motor.
The foregoing description describes a typical closed loop control system for a master-slave process. Such a closed loop control system is usually an adequate means of synchronizing master and slave speeds to a desired speed relationship. However, such a system is not usually adequate if the position of a product, such as a web which is carried by a machine driven by the master process motor, must be precisely determined with respect to the slave process, such a rotary applicator. Such control is important, for example, if the web having hot melt material applied thereon is subject to later processing which depends on accurate positional coating of the web.
Because speed is a measurement of distance or position over time, merely controlling the speed of the web will not insure accurate positioning of the web over any predetermined short time interval. Moreover, should the position of the web become progressively improperly positioned with the rotary applicator during the process, a perfect unerring control of a desired speed slave-master speed ratio will serve only to maintain this positional error. This type of positional error typically "creeps" into the system over time as an accumulated error which grows during the course of the process.
Accordingly, if the position of a product, such as a web carried by a master process machine, must be precisely maintained with respect to the slave process, such a rotary applicator, auxiliary control means must be provided in addition to those recited above. One such auxiliary control means is provided by the M-Track motor speed controller, manufactured by Fenner/Comtrex Industrial Controls of Maple Grove, Minn. which provides an auxiliary input for a photosensor (photo eye) which reads photo registration marks (I-marks) on the web. The I-marks are positioned at regular intervals along the border of the web. The controller uses this auxiliary input for varying the controller output control signal.
Auxiliary control means such as that described above include a single control loop to compare the reference speed master signal, the fixed setpoint representing a desired slave/master speed ratio, the slave speed feedback signal, and the auxiliary photosensor web position input to arrive at an appropriate output control signal to the slave process motor. The reference speed master signal, the slave speed feedback signal, and the auxiliary photosensor signal are read in real time, thereby providing a current indication of the relative speeds of the master and slave processes and the position of the web. The fixed setpoint, however, merely represents a desired slave/master speed ratio which has been determined at the initialization of the process being controlled.
The single control loop output signal is determined by comparing the real time indications of the master and slave processes and the position of the web, with the desired setpoint speed ratio. If, over the course of time, the photosensor signal indicates that the positional error has gradually and continually increased ("creeped" into the system), each time the control loop reads the photosensor signal it must correct its output signal based on the positional error indicated by the photosensor signal. Positional errors cannot be eliminated without temporarily deviating from the desired setpoint speed ratio. The desired setpoint speed ratio, however, remains unchanged.
This operation results in control loop response times for resolving the positional error which grow along with the "creeping" accumulated positional error of the web. The greater the positional error indicated by the photosensor, the greater the correction factor the control loop must arrive at to match the speed ratio setpoint. The control loop is therefore fighting itself by recognizing a positional error and then correcting its output signal in a manner which may only serve to assure the continued, gradual increase of this error. In addition to increased response time, such operation may also result in eventual instability of the system.
It is an object of the invention therefore, to provide an improved electrical control circuit for controlling the position, speed and direction of a rotary screen coater with respect to the line position, speed and direction of a moving web, which prevents "creeping" positional errors of the web from being introduced into the system, and which provides a dynamic means of continually adjusting the position, speed and direction ratio setpoint to provide for better circuit response time.